Copy transactions individually
Ctrl+C, Ctrl+V for an individual transaction.
Copy transactions when you copy a book
Open the Configuration menu (Gear button).
Select "Make a copy...".
Select "Copy Transactions".
Only book owners can copy transactions.
Copy transactions using Google Sheets
To copy transactions from another book or merge transactions into another book, use the Bkper Add-on for Google Sheets. First export transactions to a Google Sheet and then import them in the other book. See more here.